Lewis and Clark named the three rivers which
combine to form the Missouri
(Jefferson, Gallatin, and Madison rivers).

July 28, 1805: "both Capt. Clark and myself corrisponded in
opinion with rispect to the impropriety of calling either of these streams
the Missouri and accordingly agreed to name them after the President of
the United States and the Secretaries of the Treasury and state having
previously named one river in honour of the Secretaries of War and
Navy. In pursuance of this resolution we called the S.W. fork, that
which meant to ascend , Jefferson's River in honor of Thomas
Jefferson. the Middle fork we called Madison's River in honor of
James Madison and the S.E. Fork we called Gallitin's River in honor of
Albert Gallitin. the first two are 90 yards wide and the last is 70
yards. all of them with great valocity and thow out large bodies of
water" Captain Lewis